Fix exec into pods to resize volumes for multi-container pods.

The original code assumed only one container per pod.

pkg/cluster/exec.go

@@ -12,6 +12,7 @@ import (
 	"k8s.io/client-go/tools/remotecommand"
 
 	"github.com/zalando-incubator/postgres-operator/pkg/spec"
+	"github.com/zalando-incubator/postgres-operator/pkg/util/constants"
 )
 
 //ExecCommand executes arbitrary command inside the pod
@@ -28,8 +29,17 @@ func (c *Cluster) ExecCommand(podName *spec.NamespacedName, command ...string) (
 		return "", fmt.Errorf("could not get pod info: %v", err)
 	}
 
-	if len(pod.Spec.Containers) != 1 {
-		return "", fmt.Errorf("could not determine which container to use")
+	// iterate through all containers looking for the one running PostgreSQL.
+	targetContainer := -1
+	for i, cr := range pod.Spec.Containers {
+		if cr.Name == constants.PostgresContainerName {
+			targetContainer = i
+			break
+		}
+	}
+
+	if targetContainer < 0 {
+		return "", fmt.Errorf("could not find %s container to exec to", constants.PostgresContainerName)
 	}
 
 	req := c.KubeClient.RESTClient.Post().
@@ -38,7 +48,7 @@ func (c *Cluster) ExecCommand(podName *spec.NamespacedName, command ...string) (
 		Namespace(podName.Namespace).
 		SubResource("exec")
 	req.VersionedParams(&v1.PodExecOptions{
-		Container: pod.Spec.Containers[0].Name,
+		Container: pod.Spec.Containers[targetContainer].Name,
 		Command:   command,
 		Stdout:    true,
 		Stderr:    true,